Experiments described in this proposal are designed to elucidate the descending central auditory pathways which terminate in the cochlear nucleus of a primate, the bush baby (Galago crassicaudatus). In order to discover and describe these pathways, the retrograde tracer horseradish peroxidase (HRP) will be injected into first, the whole of, and then various subdivisions of the cochlear nucleus. The resulting labeled neurons in the brainstem will indicate the source of specific descending projections, but not the actual target or the pathway to that target. In order to determine the path and specific termination areas of these descending projections, tritiated amino acids and PHA-L will be injected into areas which contained HRP-labeled neurons and their course followed to the ultimate destination in terminal boutons of the descending auditory pathway. Although these methods will describe the origin, course, and destination of descending pathways to the cochlear nucleus, they cannot describe the function of these pathways. Thus, the third method of our neuroanatomical arsenal, the immunohistochemical labeling of the inhibitory neurotransmitter, GABA, will provide insight into how some of these descending pathways may influence ascending auditory afferent input arriving in the cochlear nucleus from the periphery. The experimental strategy of labeling GABA terminals before and after lesions located at the source of descending pathways to the cochlear nucleus will provide data as to which, if any, of the descending terminals may provide GABAergic input to cochlear nucleus. The long-range objective of this proposed research is to elucidate the mediating neuroanatomical connection between the central auditory system and motor output pathways capable of controlling behavior. Results of this research will provide information about descending inhibitory influences on incoming sensory neural activity significant for interpreting clinical auditory brainstem responses from patients with centrally-located pathology.